Digital to analog converters (DACs) provide a basis for design of arbitrary waveform generators, as well as a wide variety of measuring devices, and have gained widespread acceptance in broadband communications applications.
To achieve relatively high speed and large bandwidth characteristics for a DAC operating at a given frequency, the DAC may be built as a composition of a number M of sub-DACs, with each sub-DAC operating at a frequency which is reduced by a factor of M from the given frequency. Such DACs are known as composite DACs in the art, sometimes referred to as time-interleaved DACs. A block diagram of an exemplary conventional composite DAC is shown in FIG. 1 (see, for example U.S. Pat. Nos. 9,007,250, 9,973,293 and others). In use, for the DAC of FIG. 1, a digital input signal at a digital input 12 associated with a sampling frequency Fs for the DAC, is applied to a demultiplexer. The demultiplexer splits the input signal into M partial data streams. Each partial data stream has an associated sampling frequency Fs/M and is applied to the input of an associated one of the sub-DACs #1-#M. Each sub-DAC converts its received partial digital stream into an analog signal. The outputs of all sub-DACs #1-#M are connected to associated inputs of a combiner. The combiner passes over to its output, the signal from one of its inputs after another, in succession, forming in that way, a combined signal. The output of the combiner is connected to the input of a filter with a passband that corresponds to the operative band of the DAC in the first Nyquist zone. The filter smooths the signal formed in the combiner, and places that smoothed analog signal at the composite DAC output 14.
Composite DAC architectures, as just described, suffer from differences, or mismatches, in properties of the sub-DACs making up the composite DAC. The main impact of such mismatches is an appearance of spurious frequency components in the composite DAC output analog signal. The spurious frequency components are perceived as distortions of the processed signal. Those distortions diminish the spurious free dynamic range (SFDR), limiting the overall effective number of bits (ENOB) of the DAC, and should be eliminated or, at least, reduced.
There are different possible reasons for appearance of the spurious frequency components in the composite DAC output: timing misalignment, DC offset misalignment, different amplification in sub-DACs and misalignment between frequency responses hm(F) of the different sub-DACs making up the composite DAC.
The reduction of spurious frequency components in time-interleaved DAC outputs (often referred to as spurs reduction) has received reasonable amount of attention in the art. For example, U.S. Pat. No. 9,685,969 proposes a time-interleaved DAC architecture that reduces time misalignment of its sub-DACs. The U.S. Pat. No. 7,084,794 describes a method and apparatus for DAC DC offset calibration. The calibration that removes the converting elements mismatch, is treated in the U.S. Pat. No. 8,125,361. U.S. Pat. No. 8,134,486 deals with a calibration mechanism of a DAC in a sigma delta modulator. However, the problem of correction of frequency responses misalignment in multiple sub-DACs of a time-interleaved DAC, cannot be considered as solved at the present time.
In the prior art, the elimination of sub-DACs-caused frequency responses misalignment requires a calibration of the device over its full output bandwidth. The calibration typically requires preliminary measurement of the DAC features which have caused the undesirable effect. To be able to perform DAC calibration in production or operational environments, this measurement should be performed by the most simple and inexpensive measuring device, such as spectrum analyzer, producing only an improved amplitude spectrum of the processed signal.
The present disclosure describes a method and system for calibrating time-interleaved DACs which equalizes frequency response misalignments, thereby preventing spur appearance, and, importantly, which can be performed using simple spectrum analyzer as a principal measurement tool.